The present invention relates to a brake cylinder with parking brake function.
Brake cylinders of the general type under consideration function to actuate the brakes of a vehicle during service braking. Typically, compressed air is used to drive these brake cylinders, especially in commercial vehicles.
Brake cylinders of the general type under consideration may additionally take over the function of a parking brake. For a parking brake, it can be a regulatory requirement that the braking effect must be maintained even without the action of a pressurized fluid. Thus, the brake cylinder must be capable of being locked in its extended position, which position is also dependent on abrasion or wear of the brake linings, in order that the braking effect can be maintained in the unpressurized condition.
EP 0718168 A1 describes a brake cylinder for a compressed-air-actuated vehicle brake that is equipped with a releasable toothed mechanical locking device. This remains non-functional during service braking, but can be converted to locked position during auxiliary and/or parking braking. A disadvantage of this conventional brake cylinder is the relatively complicated design and the fact that the toothed mechanical locking device can be adjusted only incrementally.
EP 0377898 A1 describes a one-stage brake cylinder with a piston-rod clamping fixture. Infinitely variable clamping is made possible here by a conical clamping ring that presses on the piston rod to immobilize it in the end position (parking position) of the piston rod. An extra compressed-air line actuates the clamping ring. A disadvantage of this conventional brake cylinder is the relatively complicated design and the large radial forces produced due to clamping.
Brake cylinders of the type referred to as spring brake actuators are also known (see e.g., DE 3113586 A1). In such brake cylinders, a strong spring actuates the brake in the unpressurized condition of the spring actuator pressure chamber. To release the spring actuator, the spring is compressed by means of compressed air. Such spring actuators already, by their nature, have an integrated parking brake function. A releasing device is necessary for emergency release of the spring actuator if the compressed air fails. Spring brake actuators are relatively bulky and therefore are not particularly suitable for confined space conditions, such as exist on the front axles of commercial vehicles.